Most tissues in complex metazoans contain a rare subset of cells that,
at the single-cell level, can self-renew and also give rise to mature daughter
cells. Such stem cells likely in development build tissues and are retained
in adult life to regenerate them. Cancers and leukemias are apparently not
an exception: rare leukemia stem cells and cancer stem cells have been isolated
that contain all of the tumorigenicity of the whole tumor, and it is their
properties that will guide future therapies. None of this was apparent just
20 years ago, yet this kind of stem cell thinking already provides new perspectives
in medical science and could usher in new therapies. Today, political, religious,
and ethical issues surround embryonic stem cell and patient-specific pluripotent
stem cell research and are center stage in the attempts by governments to
ban these fields for discovery and potential therapies. These interventions
require physicians and physician-scientists to determine for themselves whether
patient welfare or personal ethics will dominate in their practices, and whether
all aspects of stem cell research can be pursued in a safe and regulated fashion.

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